Process for heat treating superconductor wire

ABSTRACT

An oxide superconductor wire is prepared by preparing a length of precursor wire for processing into an oxide superconductor wire and coating the precursor wire with an isolating layer. The coated wire is wound onto a reel in a spiralling manner, such that each turn of the spiral is in alignment with the preceding turn of the spiral along an axis perpendicular to the axis of winding. The wound precursor wire is then heated to form the oxide superconductor. The removable isolating layer is prepared by coating the wire with a solution including a metal oxide and a porosity-inducing component, and heating the coated wire so as to induce porosity and control grain size of the metal oxide so as to render the coating removable. The coating should function to isolate the overlapping turns of the wound wire from neighboring wires, so that not diffusion bonding or adherence between the turns occurs. The coating should also be readily removable because the coating can interfere with subsequent processing of the oxide superconductor wire.

[0001] This application is a continuation application of U.S. patentapplication Ser. No. 09/309,220, filed May 10, 1999, which is acontinuation of U.S. patent application Ser. No. 08/235,560, filed Apr.29, 1994 both of which are incorporated herein by reference.

OBJECT OF THE INVENTION

[0002] The invention relates generally to a system and process formanufacturing high transition temperature (T_(c)) oxide superconductorwire. The invention more particularly relates to a system and apparatusfor coating, winding and heat treating high T_(c) oxide superconductorwire.

BACKGROUND OF THE INVENTION

[0003] Many applications of the high T_(c) oxide superconductorsrequires forming the superconductor into a wire. Current processesinclude forming a precursor of the oxide superconductor or the oxidesuperconductor itself into a wire and heat treating the wire to obtainan oxide superconductor wire. Current developments in high T_(c) oxidesuperconductor processing have resulted in the manufacture ofincreasingly longer lengths of oxide superconductor wire with acceptablecurrent carrying capacity. The loading and unloading of oxidesuperconductor wire is an important step in the processing of the wire,in particular, to move the wire between wire deforming steps, such aspressing and rolling, and oxide superconductor phase-forming steps, suchas sintering and annealing. The prior art wire deforming step istypically carried out by feeding the wire from a conventional spoolthrough the deforming step and taking up the deformed wire onto a secondconventional spool. Long lengths of wire can be efficiently wound ontothe spool. However, the conventional spool is not readily adaptable foruse in furnaces and results in inefficient use of furnace space.Further, the wire tends to sag and become distorted (wavy) because ofthe coiled form of the wire on the spool. Lastly, the multipleoverlapping windings on the spool do not permit efficient oxidation andphase transformation of the oxide superconductor.

[0004] A further disadvantage to winding the oxide superconductor wireprior to heat treatment is that overlapping contact between portions ofthe wire results in diffusion bonding of the wire to itself and themandrel, thereby degrading superconducting properties and preventing theunspooling of the heat treated wire. Silver is commonly used as aprotective cladding for the oxide superconductor, in particular becausethe cladding itself is electrically conductive and does not preventoxygen diffusion to the oxide superconductor. However, even the silvercladding will diffusion bond to other portions of the silver-claddedwire which are in contact during heat treatment.

[0005] U.S. Pat. No. 5,140,006 discloses a method and apparatus forcoating a silver-cladded oxide superconductor wire with adiffusion-inhibiting material and taking up the coated wire onto aspool. Rare earth oxides are specifically disclosed as a desirablediffusion inhibiting material and no disclosure of the desirability ofremoving the material after treatment is disclosed.

[0006] It is the object of the present invention to efficiently processhigh-T_(c) superconducting oxide wire by increasing the simplicity andefficiency of the method used to load and unload wire during processing,by reducing adhesion of the wire to itself and by maximizing the use offurnace space during heat treatment.

SUMMARY OF THE INVENTION

[0007] In one aspect of the invention, an oxide superconducting wire isprepared by providing an oxide superconductor wire and applying anisolating layer to an external surface of the wire. The isolating layerincludes an isolating material and a porosity-inducing component. Thecoated wire is heated so as to induce porosity into the isolating layer,thereby obtaining a removable porous isolating layer. The coated oxidesuperconductor wire is then further processed, as required.

[0008] In another aspect of the invention, an oxide superconducting wireis prepared by providing an oxide superconductor wire and applying anisolating layer to an external surface of the wire. The isolating layerincludes an isolating material and a porosity-inducing component. Thecoated wire is wound onto a reel in a spiralling manner, such that eachturn of the spiral is in alignment with the preceding turn of the spiralalong an axis perpendicular to the axis of winding. The coated oxidesuperconductor wire is then further processed, as required.

[0009] By “oxide superconducting wire”, as that term is used herein, itis meant a wire at all stages of manufacture, but which can ultimatelybe processed into a superconductor wire. Therefore, precursor wires,which are converted into an oxide superconductor wire is deemed asuperconductor wire for the purposes of the invention. Likewise, a wirewhich is only partially converted into the oxide superconductor, orwhich contains the oxide superconductor but requires further processingto optimize the electrical properties are likewise deemed an oxidesuperconductor wire for the purposes of the invention.

[0010] By “in alignment” as that term is used herein, it is meant thateach turn of the spiral is in alignment with and substantiallycompletely overlapping with the previous turn of the spiral, such thatthe wound wire takes on a substantially two-dimensional appearance. Thewound wire is similar in appearance to a wound cassette reel of audiotape.

[0011] By “porosity-inducing component” as that term is used herein, itis meant a primarily carbon-containing material which is capable ofcombustion or thermal decomposition with very little or no residues. Thespace formerly occupied by the component, after such decomposition,provides the requisite porosity.

[0012] The oxide superconducting wire prepared by the present inventionpossesses superior isolation of successive wire layers and the spool andis capable of easy removal of the layer for further wire processing.

BRIEF DESCRIPTION OF THE DRAWING

[0013] The novel features of the invention both as to its structure andoperation is best understood from the accompanying drawings, taken inconjunction with the accompanying description, in which similarreference character refer to similar parts, and in which:

[0014]FIG. 1 is (a) a front view and (b) a side view of an apparatusused in coating and winding the precursor wire according to theinvention;

[0015]FIG. 2 is a side view of an apparatus used in coating and windingthe precursor wire according to the invention;

[0016]FIG. 3 is (a) an edge on view and (b) a side view of a reel usedin winding the wire according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0017] The applicants have discovered that coating a superconductor wirewith the isolating layer of the present invention results in superiorisolation of successive wire layers and in easy removal of the layer forfurther wire processing. The ease of coating, superiority of itsisolating properties and ease of removal provides several processingadvantages. Further, the applicants have discovered that the oxidesuperconductor wire, when wound onto a reel-like support according tothe method of the invention, can be processed more efficiently in afurnace environment, without detrimental affect to processing conditionsor final superconducting properties. The present invention permits anoxide superconductor wire to be rapidly and efficiently loaded andunloaded onto a cassette reel for furnace processing during oxidesuperconductor phase-forming steps. The wire then may be transferred toconventional spools for wire-forming steps, if desired. In addition,cassette reels are easily interfaceable with other processing equipment,in particular automated processing equipment, which improves theefficiency of the manufacturing process, if desired.

[0018] The applicants have found that use of a porous isolating layereffectively prevents bonding of the wire turns to each other, whilebeing capable of easy removal. The porosity of the layer increases theease with which the coating can be removed for the wire by reducingsurface area contact with the wire and increasing coating brittleness.The isolating material may be any material which prevents sticking ofoverlapping layers of superconductor wire and which does not poison thesuperconducting wire and degrade superconducting properties.

[0019] A variety of refractory metal oxides may be used in the practiceof the invention. The applicants have found oxides of aluminum, calcium,tantalum, magnesium, zirconium and tungsten to be particularlyeffective. Selection of a particular refractory metal oxide is based onthe ease of its removal and effectiveness in isolation of the wireturns. Magnesium oxide (MgO) is a particularly preferred metal oxide. Itis preferred to use refractory metal oxides having a relatively largeparticle size and, in particular, a particle size in the range of 1 to50 microns. Refractory metal oxide particles of small grain size promoteadherence to the wire and make it difficult to remove. The adherence isof a mechanical, rather than a chemical, nature.

[0020] The porosity of the layer improves the ease of its removalsubsequent to the heat treatment. The ability to remove the isolatinglayer after heat treatment is important because the layer may impedefurther processing of the wire. For example, where the isolating layeris not of a uniform thickness, rolling operations result in unevenstress being applied along the wire length and/or width. Porosity may beobtained by including a porosity-inducing component in the isolatinglayer which is capable of thermal decomposition or combustion withlittle or no residue. In the subsequent heat treatment, theporosity-inducing component decomposes or combusts, leaving voids in theisolating layer and thereby introducing porosity into the layer.Suitable porosity-inducing components, include but are in no way limitedto, cellulose, wood fiber, saw dust, graphite, paraffin, polypropyleneand polyethylene.

[0021] The isolating layer, including an isolating material and aporosity-inducing component, may be applied using an inert solvent asthe carrier liquid. By inert solvent, it is meant herein that theisolating material and the porosity-inducing component are stable in thesolvent with no adverse reactions between the solvent and the addedmaterials. It is further required that the porosity-inducing material beinsoluble in the carrier liquid, since a soluble material does notoccupy a significant volume in the resulting layer.

[0022] The isolating material and the porosity-inducing component areadded to an inert solvent. The resultant mixture can be a solution (ofthe isolating material only), dispersion, slurry or a suspension;however, due to the low solubility of the isolating material and theporosity-inducing component, the mixture is most typically a suspensionor slurry. The solvent is preferably volatile, so that evaporation ofthe solvent and subsequent adherence of the layer to the wire occursrapidly. The solvent is preferably a low molecular weight liquid, suchas ethanol, acetone, hexane or water.

[0023] The weight percent of added solids (isolatingmaterial+porosity-inducing component) in the mixture may be in the rangeof 5 to 25 wt %. An exemplary mixture is prepared by addingapproximately 100 g powder into 1 liter of ethanol (approx. 10 wt %).The solids range from 5 wt % to 99 wt % isolating material, the balanceporosity-inducing component. The greater the level of isolating materialin the solids, the more effective the anti-bonding effect of the layer;however, this improvement is obtained at the cost of the removability ofthe layer. A preferred process uses 50 g MgO, 50 g cellulose in 1 literof ethanol; however, ratios of 10% MgO/90% cellulose have beensuccessfully used to provide a highly releasable layer.

[0024] It may be desirable to vary the relative proportions of theisolating material and porosity-inducing component in the isolatinglayer during processing of the oxide superconductor wire, where thelayer is repeatedly applied and removed. For example, in earlyprocessing steps, where greater problems with sticking of the wire turnsis observed, it is desirable to apply an isolating layer of higherisolating material content, i.e., greater than 50% MgO. In laterprocessing steps, where bonding of the wire turns is less marked, it isdesirable to apply an isolating layer with higher levels ofporosity-inducing component to facilitate easy removal of the layer. Theappearance of the wire is also greatly improved.

[0025] The mixture may be applied to the wire in any conventionalmanner, including but not limited to, spray coating and dip coating andthe like. Spray coating includes generating droplets of the carrierliquid containing the isolating material and porosity-inducing componenttherein and impinging the droplets on the surface. Dip coating includespassing the wire through a bath which contains a mixture of theisolating material and porosity-inducing component therein. In otherembodiments of the invention, the isolating material and theporosity-inducing component may be introduced in two separateapplication steps (i.e., spray coating of first one, and then the other,component onto the layer). In such instances, the porosity-inducingcomponent should be applied closest to the wire in order to ensure easyrelease.

[0026] The method is described with reference to an apparatus which issuitable for practicing the invention. Coating and loading a wire onto areel may be accomplished from a second reel, which is useful when themethod is to be integrated with automated reel-based technology. Asshown in FIG. 1(a) and (b), an apparatus 30 includes a base 31 whichsupports a receiving reel 32 using mounting means 33 which is capable ofrotation. The receiving reel 32 is driven by a motor 34, which causesthe reel to rotate. A wire 35 is first provided on a feeding reel 36which may be rotatingly mounted on the base 31 by mounting means 37.Upon rotation of the feeding reel 36, wire 35 is unwound. Alignment ofthe receiving and feeding wheels 32 and 36, respectively, is preset toprevent twisting and distortion of the wire. The wire is loaded onto thereceiving reel 32 as a spiral with each turn of the spiral aligned withthe previous turn, so that the turns are aligned and substantiallycompletely overlapping along an axis perpendicular to the axis ofrotation. A control panel 39 controls the speed of the driving motor 34.

[0027] In preferred embodiments, an isolating layer is applied to thewire before it is taken up onto the reel 32. The isolating layerprevents overlapping layers from sticking to each other after heattreatment. To this end, the apparatus 30 includes a receptacle 38 forholding a mixture which comprises an isolating material and apore-inducing component in an inert solvent. The receptacle 38 maycontain a guide pulley (not shown) for directing the wire down into themixture contained in the receptacle and back out of the receptacletowards the reel 32. The guide pulley additionally serves as a means ofstirring the mixture during a coating operation. Some means of agitationis required to maintain dispersion of the suspended insoluble particles.Alternative means of agitation include use of a circulating pump,mechanical stirring or convection means.

[0028] In operation, one end of the wire 35 is attached to the receivingreel 32 and the motor 34 is activated. The wire 35 is unwound fromfeeding reel 36 and drawn around the pulley and into the receptacle 38.The wire contacts the mixture contained in the receptacle, therebydepositing a layer of isolating material on the wire. Before the wire iswound on the reel, the solvent dries to leave an isolating layer on thewire. Drying means (not shown) can be used to increase drying rate, ifdesired. Drying means include fans or heater or the like suitablybetween the receptacle 38 and reel 32.

[0029] As described above, coating and loading the wire onto reel 32 maybe accomplished from a cassette reel, which is useful when the method isto be integrated with automated processing. Alternatively, coating andloading the wire may be accomplished from spool to reel. To do so, themethod and apparatus are modified, as shown in FIG. 2, where likeelements are similarly numbered. A spool 40 is positioned adjacent tothe feeding reel 36. The wire 35 is fed from the spool 40 and drawn overfeeding reel 36. The wire is then processed as described above for areel to reel transfer. The spool 40 should be positioned a distance fromreel 36 so that the wire 35 is not significantly bent or distorted as itis fed out from the spool.

[0030] It is also within the scope of the invention to apply theisolating layer by feeding out the oxide superconducting wire from aconventional spool, through a receptacle containing a mixture of theisolating material and porosity-inducing component and onto a secondconventional spool. The manner of the process is readily understood withreference to FIGS. 1 and 2, above. Further, when the coating is notremoved, it can function as a desirable insulating layer. It is alsowithin the scope of the invention to spray coat the insolating layeronto the wire during the wire loading and unloading process.

[0031] It is further contemplated that the wire may take on anygeometry, including but not limited to, wires having a circular, avoid,ellipsoidal, rectangular, square and polygonal cross-section. The wiremay have a width to thickness ratio in the range of 1 to 1000.

[0032] The reel which is used in the above method may be made from amaterial capable of withstanding high furnace temperatures, such asnickel alloys and stainless steel. With reference to FIG. 3, the reelincludes a central mandrel 42 and two opposing end plates 44 and 46,respectively. The mandrel 42 may be of substantially the same width asthe wire. The end plates preferably contain apertures 48 to allow thepassage of oxygen and/or other processing gases. Alternatively, the reelis made from components capable of assembly and disassembly. In thiscase, the end plates 44 and 46 are removed from the mandrel 48. Themandrel is a compressible three-piece mandrel which permits it to beremoved from the spiral core. The pancake-like wire spiral can be placedon a flat ceramic sheet and heat treated without the reel. It may bepreferred to use a retaining ring around the outer circumference of thespiral wire in order to retain its shape during heat treatment. Whenreel is remove prior to heat treatment, it is not required to be madeout of special high temperature materials. The reel can be made of anymaterial without regard to high temperature stability. Aluminum isacceptable.

[0033] Subsequent to winding the wire onto the reel, the wire is readyfor heat treatment. Such heat treatment includes forming the oxidesuperconductor phase or optimizing the superconducting properties of thewire. The heat treatment may be carried on the reel, or one or moreplates of the reel may be removed prior to heat treatment. During heattreatment, the isolating layer prevents bonding of the neighboringlayers. The isolating layer of the invention permits complete overlap ofthe wire turns without sticking, which is not possible in the prior art.

[0034] The isolating layer may be easily removed by passing it under astream of water, or against an abrasive surface. Ultrasonification willalso remove the layer. Other conventional means of removing coatings arewithin the scope of the present invention.

[0035] The method of the present invention can be used to process anyoxide superconductor wire. By way of example, and in no way limiting,the oxide superconductor may include rare earth barium cuprates, bismuthstrontium calcium cuprates and thallium barium calcium cuprates. Theoxide superconductor wires are typically processed with silver or othermalleable, inert metal to impart desirable mechanical properties to thewire, which is otherwise too brittle to function as a wire.

[0036] The wire used in the invention may be a precursor to an oxidesuperconductor, such as the metallic alloys disclosed in U.S. Pat. No.4,826,808 to Yurek et al., herein incorporated by reference, which isthen oxidized to form the desired oxide superconductor. Alternatively,the wire may include an oxide superconductor which is subjected tofurther heat treatment to optimize the formation and properties of theoxide superconductor. The wire preferably contains a malleable, butinert, metal for improved formability and mechanical flexibility. Thepreferred metal is silver. Oxide superconductor wires suitable for usein the method of the present invention are described in “Critical IssuesOPIT Processing of High-T_(c) BSCCO Superconductors” by Sandhage et al.(JOM 43(3), 21-25 (1991)), herein incorporated by reference.

[0037] Other embodiments of the invention will be apparent to thoseskilled in the art from a consideration of this specification orpractice of the invention disclosed herein. It is intended that thespecification and examples be considered as exemplary only, with thetrue scope and spirit of the invention being indicated by the followingclaims.

What is claimed is:
 1. In a method of preparing a metal tube for alength of superconducting wire comprising inserting a superconductingbase material into the metal tube, the metal tube having an outersurface, and packing or coiling the metal tube so that a portion of theouter surface of the metal tube would contact another surface, wherebyheating the superconducting base material and metal tube to atemperature that forms of the superconducting phase of thesuperconducting base material, the temperature being sufficiently closeto the melting point temperature of the metal tube, would cause theportion of the outer surface of the metal tube to adhere to the othersurface, the improvement comprising: coating the portion of the outersurface of the metal tube with powder material particles suspended in aneffectively liquid component before the packing or coiling so that theportion of the outer surface of the metal tube does not contact theouter surface after the packing or coiling, the powder material having amelting point temperature higher than the melting point temperature ofthe metal tube, and the liquid component evaporating or burning awaycompletely at the temperature that forms the superconducting phase ofthe superconducting base material.
 2. The method according to claim 1,wherein the powder material is a metal oxide.
 3. The method according toclaim 1, wherein the coating is such that the particles do not touch theother surface after the packing or coiling.
 4. The method according toclaim 2, wherein the coating is such that the particles do not touch theother surface after the packing or coiling.
 5. The method according toclaim 1, wherein the liquid component comprises a solvent admixed with apolymer.
 6. The method according to claim 1, wherein the liquidcomponent comprises a solvent admixed with a polymer.
 7. The methodaccording to claim 3, wherein the solvent comprises at least one ofacetone, alcohol or toluene.
 8. The method according to claim 6, whereinthe solvent comprises at least one of acetone, alcohol or toluene. 9.The method according to claim 3, wherein the polymer comprises polyvinylbutyral.
 10. The method according to claim 8, wherein the polymercomprises polyvinyl butyral.
 11. The method according to claim 9,wherein the polyvinyl butyral comprises from 0.1 to 10 percent by weightof the liquid component.
 12. The method according to claim 10, whereinthe polyvinyl butyral comprises from 0.1 to 10 percent by weight of theliquid component.
 13. The method according to claim 2, wherein the metaloxide powder material comprises at least one of Al₂O₃, Cr₂O₃,Cu_(x)O_(y), Ni_(x)O_(y), Zn_(x)O_(y), Z_(x)O_(y) or Ta_(x)O_(y). 14.The method according to claim 12, wherein the metal oxide powdermaterial comprises at least one of Al₂O₃, Cr₂O₃, Cu_(x)O_(y),Ni_(x)O_(y), Zn_(x)O_(y), Zr_(x)O_(y) or Ta_(x)O_(y).
 15. The methodaccording to claim 2, wherein the metal oxide powder material comprisesZr.
 16. The method according to claim 15, wherein the metal oxide powdermaterial comprises Zr.